Community-Acquired Pneumonia (CAP): This is the most likely diagnosis due to the presentation of pneumonitis, which is often caused by bacterial or viral infections. The left upper lobe involvement is consistent with CAP, and wheezing could be present due to the inflammatory response and airway irritation.

Asthma: Asthma can cause wheezing and, if severe, can lead to pneumonitis due to airway inflammation and hyperreactivity. The left upper lobe involvement might be due to a localized allergic reaction or irritant exposure.
Chronic Obstructive Pulmonary Disease (COPD) Exacerbation: COPD can cause wheezing and, during exacerbations, can lead to pneumonitis, especially if there is a superimposed infection.
Influenza: Influenza can cause pneumonitis and wheezing, especially in individuals with underlying respiratory conditions. The left upper lobe could be involved, depending on the severity and the individual's response to the infection.

Pulmonary Embolism: Although less likely, pulmonary embolism can cause sudden onset of symptoms, including wheezing and pneumonitis if there is infarction of lung tissue. It's crucial to consider this diagnosis due to its high mortality rate if untreated.
Tuberculosis (TB): TB can cause pneumonitis and wheezing, especially if it involves the upper lobes. It's essential to consider TB, especially in individuals with risk factors such as immunocompromised status or exposure history.
Pneumocystis jirovecii Pneumonia (PCP): In immunocompromised patients, PCP can cause pneumonitis and wheezing. It's a critical diagnosis to consider due to its severity and the need for specific treatment.

Eosinophilic Pneumonia: This rare condition can cause pneumonitis and wheezing due to eosinophilic infiltration of the lungs. It might be considered in cases where common causes have been ruled out and there are eosinophilia or other suggestive findings.
Hypersensitivity Pneumonitis: This is a rare condition caused by an allergic reaction to inhaled substances, leading to pneumonitis and potentially wheezing. It's essential to consider this in individuals with occupational or environmental exposures.
Lymphangitic Carcinomatosis: Although rare, this condition, which involves metastatic cancer spreading to the lymphatics of the lungs, can cause pneumonitis and wheezing. It's a diagnosis to consider in patients with known cancer or those at high risk.